Curable film-forming coating compositions have long been used in automotive and other industrial manufacturing fields. Curable compositions, that is, compositions that form hard surface protecting layers by chemical crosslinking offer better protection (as opposed to lacquers) to substrates because of their ability to resist damage by solvents, acids, and other chemicals, and effects of weathering. In order to cure by crosslinking, curable coating compositions contain chemical groups that are reactive under curing conditions to effect the required crosslinking.
A variety of crosslinking agents are utilized in curable compositions, including polyacids, aminoplasts, and polyisocyanates. Each type of crosslinking agent undergoes different chemical reactions with resinous binders, depending on the respective functionality of the crosslinking agents and binders. Each crosslinking agent also has its own advantages and drawbacks associated with it.
Among the available crosslinking agents are active ester functional materials. An active ester functional material is an ester functional compound having at least one hetero atom (nitrogen, oxygen, halogen, and the like) attached to the alpha carbon, rendering the ester group more reactive than an unsubstituted ester. By "more reactive" is meant that active esters may undergo transesterification or amidation more readily than unsubstituted esters, i. e., at lower temperatures or even at room temperature. The greater the number of hetero atom substitutions, the greater the reactivity.
Austrian Patent No. 361,587 discloses an electrodepositable curable composition comprising an active ester functional material (e. g., a polyisocyanate reacted with ethyl lactate) and an amino functional resinous binder, which may undergo amidation during cure. The composition disclosed is curable only at elevated temperatures.
Aminoplasts are also curable at elevated temperatures. Compositions containing aminoplasts may also contain free formaldehyde which is used in the preparation of aminoplasts, which may be undesirable.
Coating compositions containing free polyisocyanates can be cured at room temperature and yield coatings having urethane linkages which are highly resistant to chemical attack, but polyisocyanates having free isocyanate functionality are known to require special handling and environmental precautions. Curable compositions containing blocked polyisocyanates often require elevated temperatures to deblock the isocyanate groups and effect cure.
It would be desirable to provide compositions that may be used as crosslinking agents in curable film-forming compositions, wherein the compositions are formaldehyde free and curable at temperatures as low as ambient.